Micro-colloid thruster system

ABSTRACT

A micro-colloid thruster system may be fabricated using micro electromechanical system (MEMS) fabrication techniques. A beam of charged droplets may be extracted from an emitter tip in an emitter array by an extractor electrode and accelerated by an accelerator electrode to produce thrust. The micro-colloid thruster system may be used as the main propulsion system for microspacecraft and for precision maneuvers in larger spacecraft.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional PatentApplication Ser. No. 60/192,647, filed on Mar. 27, 2000.

ORIGIN OF INVENTION

[0002] The invention described herein was made in the performance ofwork under a NASA contract, and is subject to the provisions of PublicLaw 96-517 (35 U.S.C. 202) in which the Contractor has elected to retaintitle.

BACKGROUND

[0003] Microspacecraft, also referred to as micro-, nano-, orpicosatellites, depending on their size, may range in mass from under akilogram to the tens of kilograms. Microspacecraft architectures arebeing considered for scientific exploration missions beyond earth orbitas well as near-earth military missions.

[0004] The use of multiple microspacecraft may increase survivability ofa mission by providing redundancy and/or increase the overall capabilityof the system. For example, an antenna array including multiplemicrospacecraft, each equipped with its own antenna, may enable veryhigh resolution observations of Earth. The reliability of the system mayalso be increased because the use of multiple microspacecraft providesfunctional redundancy, and loss of one, or even a few, microspacecraftin the array may not represent a catastrophic failure.

[0005] Making microspacecraft viable for such applications requiressubstantial reductions in size, weight, and power for each spacecraftsubsystem. For example, micropropulsion systems capable of thrust levelsin the milli-Newton range and capable of impulse bits as little as 10⁻⁶N*s may be required in order to perform repositioning maneuvers with thedegree of precision necessary for such miniature spacecraft.

SUMMARY

[0006] A micro-colloid thruster module is described. According to anembodiment, the module may be fabricated using silicon processingtechniques, including micro electromechanical system (MEMS) techniques.The thruster module may include a number of emitters arranged in anarray. Each emitter includes a propellant inlet for receiving a liquidpropellant, e.g., a doped glycerol, an emitter tip, an extractorelectrode, and an accelerator electrode. A voltage applied to theextractor electrode produces an electric field at the emitter tip,causing the tip to emit a beam of charged droplets.

[0007] A voltage converter converts a bus voltage to an acceleratorvoltage, which may be about 2 kV to 20 kV. The accelerator voltage isapplied to the accelerator electrodes to accelerate the charged dropletsas they exit the module.

[0008] In an embodiment, the voltage converter utilizes a transformerand a stacked array of capacitors and diodes to increase the bus voltageto the accelerator voltage. In another embodiment, an array ofaccelerator electrodes in an accelerator section step up the voltage tothe accelerator voltage.

[0009] A controller may be provided to selectively activate emitters inthe module in order to control the direction and amount of thrust. Thethruster may have dimensions of on the order of about 0.1 to 1.0 cm, andprovide thrust up to about 50 μm and impulses of about 500 seconds to2000 seconds.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a cross-sectional view of an emitter according to anembodiment.

[0011]FIG. 2 is a perspective view of a micro-colloid thruster systemaccording to an embodiment.

[0012]FIG. 3 is a cross-sectional view of an emitter with an acceleratorelectrode according to an embodiment.

[0013]FIG. 4 is a perspective view of a thruster module according to anembodiment.

[0014]FIG. 5 is a plan view of an emitter/extractor structure in athruster module according to an embodiment.

[0015]FIG. 6 is a schematic diagram of a power conditioning circuitaccording to an embodiment.

[0016]FIG. 7 as a plan view of an emitter including an acceleratorelectrode array according to an embodiment.

[0017]FIG. 8 is a schematic diagram of an accelerator electrode arraywith variable length electrodes according to an embodiment.

[0018]FIG. 9 is a timing diagram for voltage pulses in acceleratorelectrodes in an accelerator electrode array according to an alternateembodiment.

[0019]FIG. 10 is a schematic diagram of a charged droplet beingaccelerated by adjacent electrodes in an accelerator electrode arrayaccording to an embodiment.

[0020]FIG. 11 is an exploded perspective view of an emitter assembly inthe thruster module according to an embodiment.

[0021]FIG. 12 is a partial cross-section of a Field Emitter Array (FEA)cathode according to an embodiment.

DETAILED DESCRIPTION

[0022] A colloid thruster device provides a beam of charged liquiddroplets to produce thrust. The beam of charged droplets may beaccelerated electrostatically. A micro-colloid emitter 100 for use in athruster module according to an embodiment includes a propellant inlet102 located below an emitter tip 104, as shown in FIG. 1. A beam 106 ofcharged droplets of a liquid propellant may be extracted from theemitter 104 by an electric field generated between the emitter tips byan extractor electrode 108.

[0023] In an embodiment, glycerol may be used as a propellant. Theglycerol propellant may be doped with a salt, such as ,odium iodine, toincrease its charge carrying capacity. An electric field on the order of10⁶ V/cm may be applied at the emitter tips 104 to extract the chargeddroplets from the propellant inlet 102.

[0024]FIG. 2 illustrates a micromachined thruster module 200 including afour-sided emitter array 202. The thruster module is provided on apropellant tank 204, which may include a capillary feed networksupplying the emitters in the array. The thruster module may operate ona low bus voltage, e.g., about 3-5 volts. The voltage supplied by thebus may be increased to a voltage sufficient to extract and acceleratecharged droplets from the emitter.

[0025] A controller 210 controls one or more emitters in the thrustermodule 202 to eject beams of charged droplets in order to producethrust. The charge on the droplets in the beams may be neutralized inorder to prevent charging the adjacent spacecraft surfaces. Beamneutralization may be achieved using a Field Emitter Array (FEA)cathode. The FEA structure 206 may be fabricated on a separate chip andbonded to the thruster module after fabrication.

[0026] The thruster module 202 may be fabricated using microelectromechanical system (MEMS) fabrication techniques. MEMS fabricationtechniques use the processes and materials of microelectronics, e.g.,batch processing of silicon wafers, to construct miniaturized systemsthat include both electrical and mechanical components. Mechanicalcomponents in MEMS, like transistors in microelectronics, produce micronsized features in numbers ranging from a few to millions.

[0027] In an embodiment, each emitter in the thruster module includes apropellant inlet 302, emitter tip 304, an extractor electrode 306, andan accelerator electrode 308, as shown in FIG. 3. The emitter tip 304may be formed as a slit having a width of about 1 μm and a depth ofabout 30 μm.

[0028] The emitter tip 304 and the extractor electrode 306 may be formedon a Silicon-on-Insulator (SOI) silicon wafer 400 using a deep reactiveion etching (DRIE) system. A cover wafer 406 may be anodically bonded tothe emitter/extractor structure to seal the structure and the liquidpropellant flow channels, as shown in FIG. 4. The cover wafer may be aglass, such as, for example, Pyrex®, a borosilicate glass productdeveloped by Corning Incorporated. The resulting structure is sandwichedbetween two silicon chips 404, each of which include thin-film depositedaccelerator electrodes 406.

[0029] The extractor electrode 306 may be spaced apart from the emittertip 304 by a gap of about 1 μm. Charged droplets may be extracted fromthe emitter tip by applying a voltage of about 100 V to the extractorelectrode 306, which generates a field of about 10⁶ V/cm at the emittertip 304. A voltage of about 10 kV may be applied to the acceleratorelectrodes 406, causing the emitted charged droplet 408 to accelerate asit exits the thruster module. The accelerator electrodes may be spacedapart from the emitter by a channel about 5 mm wide, resulting in afield strength at the tip of about 2 V/cm at 10 kV.

[0030] The emitters in the thruster module may be arranged in an array,with emitters pointing in four directions, as shown in FIG. 5.Approximately 2 μN of thrust may be generated per tip, with about 12 mWof power required per tip. An emitter array including 25 tips on a sidemay produce a thrust level of about 50 μN at a power level of 0.3 W.Multiple parallel tip and gate structures may be fabricated, all feedinginto the same accelerator channel.

[0031] A lightweight, low current, 10 kV power supply may be constructedusing a circuit 600 including a hybrid stacked array of capacitors 602and diodes 604, as shown in FIG. 6. In this circuit, application of analternating voltage (V_(ac)) to the input of the array causes the diodes604 to charge the capacitors 602 in the negative half of each AC cycle.This forces the capacitors into a series connection producing a higheroutput voltage, V_(o) (≈nV_(ac)) in the positive half of the cycle. Forexample, a micro-transformer may be used to convert the 5 V bus voltageto an input voltage (V_(ac)) of 1 kV, and a ten-stage stack can producea voltage (V_(o)) of 10 kV from the 1 kV input voltage.

[0032] The emitter/extractor assembly of the thruster module may befabricated into an SOI wafer that includes a top 30 μm thick layer ofsilicon, a middle layer of 1 μm thick silicon dioxide, and a bottomlayer of 400 μm thick silicon. The emitter/extractor geometry of thedevice may be etched into the top 30 μm layer of silicon using a DRIEsystem that anisotropically etches away the silicon. A masking layercomposed of a thin layer of photoresist may be used to define thedesired structural features. Aspect ratios of about 30 to 1 may beobtained with this method, and feature sizes as small as 1 μm wide by 30μm tall may be formed.

[0033] The underlying silicon dioxide layer of an SOI wafer may act asan etch stop to the top layer etching step. This ensures that the etchedstructures on neighboring chips on the wafer have a uniform depth. Thesilicon dioxide layer also provides electrical isolation between thebiased silicon structures that have been etched.

[0034] Propellant and electrical vias in the top 30 μm silicon layer maybe etched from the backside of the wafer using the DRIE system throughthe 400 μm bulk silicon layer. A Reactive Ion Etch (RIE) system may beused to etch the silicon dioxide layer. Vias are etched through thesilicon dioxide layer to enable electrical contact to the extractorelectrodes. A Pyrex® cover wafer may be anodically bonded to the siliconwafer. Anodic bonding allows for low temperature (<400° C.) hermeticseals between the SOI wafer and the cover wafer. The use of the coverwafer places the emitter/extractor structure into the center of theassembly, between the outer chips including the accelerator electrodes,thereby providing sufficient spacing in the vertical direction to avoidbeam impingement over the 5 mm extractor/accelerator electrode gap. Theaccelerator electrodes 406 may be metal deposited onto the silicon chips404, which are subsequently bonded to the SOI/Pyrex® assembly of thethruster module. The silicon chips 404, which include the acceleratorelectrodes 406, may be epoxy bonded to the SOI/Pyrex® assembly. Viasetched into the silicon chips 404 overlap corresponding vias etched intothe SOI wafer to allow for electrical contact with of the extractorelectrodes 306. The emitter may be contacted through the electricallyconductive propellant fluid.

[0035] In another embodiment, a multi-stage accelerator system, as shownin FIG. 7, is used to increase the bus voltage. The propellant inlet702, emitter tip 704, and extractor electrode 706 may be arranged asdescribed above. A conical multi-stage accelerator section 708accelerates the beam of charged droplets. The accelerator electrodes 710may be thin metal films about 20 μm wide that are deposited into thechannel on the insulator layer of the SOI wafer. Electrodes in theaccelerator section 708 may be separated by a gap of about 20 μm.

[0036] A pulsed DC voltage with a maximum amplitude of 100 V appliedbetween electrode pairs may be phased so that a droplet travelingdownstream through the accelerator section 708 always experiences anaccelerating electric field. Applying a different voltage between anytwo adjacent accelerator electrodes 710 creates fringing electricfields. Any charged droplet caught in the fringing field accelerates dueto the resulting electrostatic force. When a properly phased set ofvoltages are applied between electrodes 710 of the accelerator, adroplet can continue to accelerate as it passes through eachinter-electrode gap.

[0037] The increase in speed of the droplets through the array causes aphase change between the location of the droplet and the timing of theapplied voltage. In one embodiment, variable length electrodes 800 areused to compensate for this phase change, as shown in FIG. 8. In anotherembodiment, the accelerator electrodes have the same length, but avariable timing delay, such as that shown in the timing diagram of FIG.9, to compensate for the phase change.

[0038] The directional force of the fringing field influences the flowof an off-axis charged droplet through an array. The horizontalcomponent 1000 of the field points in the same direction throughout theaccelerator section, accelerating the droplet through the array. Thevertical component of the field points towards the accelerator channelcenterline 1002 in the first half of the gap and away from thecenterline 1002 in the second half of the gap, as shown in FIG. 10. Thedirection of the vertical field results in a focusing or de-focusingforce for the charged droplets. If the two vertical vectors are equal insize, the net force is zero. However, any non-zero net vector may resultin unwanted collisions with the accelerator chamber walls. The use ofrectangular waveforms over sine waves may be make it easier to maintaina zero net vector.

[0039] In an embodiment, 100 electrodes may be used to produce a totalvoltage drop of 10 kV. The overall length of the accelerator array maybe about 4 mm.

[0040] Apart from accelerating the droplet beam, the acceleratorelectrodes may also prevent beam de-focusing in the vertical direction.In order to provide beam focusing in the horizontal direction,electrostatic guide electrodes 712 (FIG. 7) may also be deposited intothe accelerator section 708, both on the exposed silicon dioxide surfaceof the silicon wafer and on the Pyrex® wafer. These guide electrodes runthe length of the channel and may be charged positively. Differentvoltages may be applied to the guides on both sides of the channel toprovide thrust vectoring.

[0041] The use of linear arrays of emitter tips allows for theactivation of only certain segments of the array, in effect causing“electrostatic gimballing” of the thruster. Varying the potentialapplied to the two guide electrodes may further amplify vectoring of thecharged droplet beam, thus avoiding the necessity of mechanical gimbals.

[0042] The thruster module may be divided into two chips 1100, 1102, asshown in FIG. 11. A first set of accelerator electrodes 1104 may beprovided on the SOI chip 1100, which includes the propellant inlet 1110,emitter and extractor electrodes. A second set of accelerator electrodes1120 may be provided on a Pyrex® cover chip 1102, which is anodicallybonded to the SOI chip 1100. The chips may be fabricated using MEMStechniques. The accelerator electrodes 710 and guide electrodes 712 inthe accelerator section may be deposited as patterned metal layers usingan e-beam evaporator and photolithography techniques. These metal layersmay be deposited onto the exposed silicon dioxide areas after DRIE ofthe top 30 μm layer.

[0043] A Field Emitter Electrode (FEA) cathode 1200 may be provided onthe thruster module to neutralize the beam of charged droplets. The FEAcathode 1200 may have a packing density of greater than 10 ⁷ tips/cm²(one tip 1202 is shown in FIG. 12) and operating voltages below 50 V.Different types of cathode materials may be used, including, forexample, HfC and ZrC cathodes and Vertical Current Limiting (VECTL)architectures. The cathode may be integrated with Cathode Lens and IonRepeller (CLAIR) grids, among other, to reduce tip sputter erosion.

[0044] A micro-colloid thruster according to an embodiment fits on achip approximately 0.5×0.5×0.1 cm³. The thruster may require about athird of a Watt of power, provide about 50 μm thrust, and generatepulses of from about 500 seconds to 2000 seconds. The controller 210 maycontrol the amount and direction of thrust by selectively activatingemitters in the array. The controller 210 may also control the directionof thrust by applying different voltages to the guide electrodes 712 inthe accelerator section 708.

[0045] A micro-colloid thruster system according to the variousembodiments may be used as the main propulsion system inmicrospacecraft. The thruster system may also be used in largerspacecraft for precise maneuvering operations. These operations mayinclude, for example, fine attitude control, orbit maintenance (dragmakeup), and formation operations involving several spacecraft.

[0046] A number of embodiments have been described. Nevertheless, itwill be understood that various modifications may be made withoutdeparting from the spirit and scope of the invention. Accordingly, otherembodiments are within the scope of the following claims.

1. A method for producing thrust, comprising: producing an electricfield at an emitter tip with an extraction voltage; extracting a beam ofcharged droplets from the emitter tip; converting a bus voltage in arange of about 3 V to about 5 V to an accelerator voltage in a range ofabout 2 kV to about 20 kV; and accelerating the charged droplets in thebeam with the accelerator voltage to produce thrust.
 2. The method ofclaim 1, further comprising converting the bus voltage to the extractionvoltage.
 3. The method of claim 1, wherein the extraction voltage is ina range of about 100 V to about 1 Kv.
 4. The method of claim 1, whereinthe thrust is about 2 μN.
 5. The method of claim 1, further comprisingmaintaining the thrust for about 1000 seconds.
 6. The method of claim 1,wherein the charged droplets comprise a glycerol doped with a dopantthat increases charge carrying capacity.
 7. The method of claim 1,further comprising providing a plurality of emitters in an array, eachemitter including an emitter tip.
 8. The method of claim 7, furthercomprising controlling the amount of thrust by selectively activatingemitters in the array.
 9. The method of claim 7, further comprisingcontrolling a direction of thrust by selectively activating emitters inthe array.
 10. The method of claim 1, further comprising neutralizingthe beam of charged droplets with a Field Emitter Array cathode.
 11. Themethod of claim 1, further comprising: providing a plurality ofaccelerator electrodes in an array, said array including a firstaccelerator electrode and a plurality of downstream acceleratorelectrodes; applying a first voltage to the first accelerator electrode;and increasing the first voltage to the accelerator voltage using thedownstream accelerator electrodes.
 12. The method of claim 11, furthercomprising: providing guide electrodes in the accelerator electrodearray; and controlling a direction of the beam of charged droplets byapplying a voltage to each of the guide electrodes to control.
 13. Amicro-colloid thruster system comprising: a plurality of emitters, eachemitter including a propellant inlet adapted to receive a liquidpropellant, an emitter tip, and an extractor electrode adjacent to theemitter tip and operative to provide a voltage sufficient to extract abeam of charged droplets of the liquid propellant from the emitter tip;a bus operative to supply a bus voltage in a range of about 3 V to about5 V; a voltage converter operative to increase the bus voltage to anaccelerator voltage in a range of about 2 kV to about 20 kV, and anaccelerator electrode operative to accelerate the beam of chargeddroplets with said accelerator voltage.
 14. The thruster system of claim13, further comprising a controller operative to activate one or moreemitters.
 15. The thruster system of claim 13, wherein a plurality ofthe emitters are arranged in different directions.
 16. The thrustersystem of claim 15, wherein a plurality of said emitters are provided oneach side of a four-sided array.
 17. The thruster system of claim 13,wherein the voltage converter comprises: a transformer to convert thebus voltage to an intermediate voltage; and a stacked array ofcapacitors and diodes to convert the intermediate voltage to theaccelerator voltage.
 18. The thruster system of claim 13, wherein thevoltage converter includes an accelerator section comprising: a firstaccelerator electrode; a plurality of intermediate acceleratorelectrodes, each intermediate accelerator electrode operative togenerate a higher voltage than the adjacent upstream acceleratorelectrode; and a terminal accelerator electrode operative to generatethe accelerator voltage from the adjacent upstream intermediateaccelerator electrode.
 19. The thruster system of claim 18, wherein theaccelerator section further comprises two or more guide electrodesoperative to guide the charged droplets through the accelerator section.20. The thruster system of claim 19, further comprising a controlleroperative to control a direction of the beam of charged droplets bycontrolling a voltage on each of the guide electrodes.
 21. The thrustersystem of claim 13, wherein the plurality of emitters, bus, voltageconverter, and accelerator electrodes are provided in a microelectromechanical system (MEMS) thruster module.
 22. The thruster systemof claim 21, wherein the MEMS thruster module comprises a semiconductormaterial.
 23. The thruster system of claim 22, wherein the semiconductormaterial comprises silicon.
 24. The thruster system of claim 13, whereinthe thruster module has dimensions on the order of about 0.1 cm to about1.0 cm.
 25. A spacecraft comprising: a propellant tank operative tostore a glycerol liquid propellant doped with dopant that increasescharge carrying capacity; a micro electromechanical system (MEMS)thruster module including a plurality of emitters, each emitterconnected to the propellant tank by a feed line and operative to emit abeam of charged droplets of the liquid propellant which provides about 2μN of thrust; and a Field Emitter Array cathode operative to neutralizethe beam of charged droplets.
 26. The spacecraft of claim 25, whereinthe thruster module has dimensions on the order of about 0.1 cm to about1.0 cm.
 27. The spacecraft of claim 25, wherein the spacecraft is amicrospacecraft.
 28. The spacecraft of claim 25, wherein the thrustermodule is operative to provide attitude control for the spacecraft. 29.A method for fabricating a thruster module, comprising: etching aplurality of emitters in a first substrate, each emitter including anemitter tip and a propellant inlet; etching an extractor electrodestructure adjacent to each emitter tip in the first substrate;depositing an accelerator electrode on a second substrate; and bondingthe first and second substrates in a thruster module assembly.
 30. Themethod of claim 29, wherein etching the emitters and extractor electrodestructures comprises deep reactive ion etching.
 31. The method of claim29, wherein the first substrate comprises a silicon layer in aSilicon-on-Insulator (SOI) substrate.
 32. The method of claim 29,wherein the second substrate comprises a cover wafer.
 33. The method ofclaim 32, wherein the cover wafer comprises a borosilicate glass. 34.The method of claim 29, wherein the accelerator electrode on the secondsubstrate is spaced apart from a extractor electrode structure on thefirst substrate by about 5 mm in the thruster module assembly.
 35. Themethod of claim 29, wherein the second substrate comprises asemiconductor material.
 36. The method of claim 29, further comprising:depositing a first plurality of accelerator electrodes on the firstsubstrate; and depositing a second plurality of accelerator electrodeson the second substrate.
 37. The method of claim 29, wherein the emittertip has a width of about 1 mm.